1. Field of the Invention
The present invention relates to a power amplifier formed by a BiFET process and, more particularly, to a power amplifier capable of realizing equivalent power characteristics of a HBT (Heterojunction Bipolar Transistor) power amplifier while obtaining low noise characteristics.
2. Background Art
GaAs-FET (Field Effect Transistor) power amplifiers have a negative threshold voltage and, therefore, have a drawback of requiring a negative gate bias voltage. In contrast, GaAs-HBT (Heterojunction Bipolar Transistor) power amplifiers require no negative gate bias voltage, being capable of single power supply operation and having more uniform device characteristics in comparison with FET power amplifiers. For this reason, use of GaAs-HBT power amplifiers in CDMA (Code Division Multiple Access) portable telephones, wireless LAN (Local Area Network) devices, etc., has been markedly increased.
A BiFET process for making a FET together with a HBT on a single substrate has recently been applied to products. Ordinarily, in a GaAs BiFET process, an HBT and a depletion mode FET (that is normally on) are mounted on a single substrate. Further, a process in which an enhancement mode FET (that is normally off) is made in addition to an HBT and a depletion mode FET on a single substrate has recently been reported in a learned society (IEEE: Radio Frequency Integrated Circuits Symposium 2008).